Abstract. The purpose of this study was to ask if cannabis growers could increase profitability by increasing light intensity far above conventional levels. We also look for evidence that the premium paid for specialty grow lights is worth it when compared to much cheaper, general application, broad-spectrum (white) lights. Then, we discuss the return on investment associated with using more electricity in order to increase light intensity and yields. Finally, when comparing lighting options, we argue that growers and policy makers should focus on $/μmols not $/watts. The experiment includes hundreds of cannabis plants. As far as we know, this research represents the first university study of its kind, and the results challenge industry conventions. We find that yields increase linearly with light intensity up to at least 1500 μmols/ m2· s, which is at least twice the intensity that is most commonly used by cannabis growers. We also find no evidence that spectrum variations across our sample of specialty horticulture lights significantly affect yields versus general purpose broad-spectrum lights. Finally, the value of the gain in yields from increasing light intensity far exceeds the cost of additional electricity for all the intensity ranges that we considered.